One of the problems with long term, implantable biomedical devices arises from the response of the body to one or more of the materials that are used to construct the device, or the degradation of the material due to the body environment. This problem has led to advances in the materials used for mechanical devices that replace bones, joints, and teeth, as well as coronary devices such as arterial stents or artificial heart valves. Biomedical devices gained an entirely new level of complexity with the advent of the microchip which adds electrical interaction with the cells to the previously utilized mechanical interaction. Unfortunately, direct long-term interaction for the microchip with the body is difficult as most of the materials used in microchips are either chemically reactive, toxic, or both, and must be hermetically sealed to maintain a degree of biocompatibility. This separation between the electrical elements of the biomedical device requires use of a transducer, an object which transforms the interaction between the body and a specific material, the analyte, into another signal which can interact with electronics. One popular transducer for both biosensors and electrical stimulation devices is the utilization of an extremely conductive material, otherwise known as an electrode.
Like every other material utilized in long term implantation, electrodes must be made of a non-corrosive, highly conductive material that does not cause adverse body reactions, is thromboresistant, and has good material durability and strength. Noble metals like gold, platinum, and iridium have been traditionally used in many electrically-active biomedical devices. Transmission electrodes that activate action potentials in electrically active cells, like the ones used in implantable heart pacemakers, have longevity in vivo, but these same electrodes experience many problems when miniaturized for smaller microelectrode array devices. This problem is most exemplified by the generation of Faradaic reactions at the surface of the electrode that produce harmful reactive species which interact with tissue near the electrode and lead to an inflammatory response. Faradic reactions are generated either due to the electrochemical interaction between the electrode and the electrolyte, but can also be dependent on the large charge injections required for cellular membrane depolarization which pass through a small geometrical surface area (i.e., high current density). The ideal electrode would not produce Faradaic reactions and have a large geometrical surface area to allow for large charge injection.
An example is the brain machine interface (BMI), which offers therapeutic options to relieve many individuals suffering from central nervous system (CNS) or peripheral nervous system (PNS) disabilities due to disease or trauma. The central component of the BMI system is the neuronal prosthetic which interacts with the body's electrophysiological signals. Implantable neuronal prosthetics have the ability to not only receive electrical signals directly from neurons or muscles, but they can deliver electrical signals to these same cells and provide a means for a closed loop BMI systems. These devices are unfortunately still regulated to experimental BMI systems due to a severe long term in vivo reliability issue. Device failure over time is thought to arise from lowered material biocompatibility which activates the immune response of the body, or failure can arise due to deterioration of the electrical interaction point due to Faradaic reactions caused by the high current densities needed for neuron action potential activation. The solution to this problem is to find materials that are physically and chemically resilient, biocompatible, and have great electrical properties.
Graphene is a two-dimensional (2D) monoatomic layer of graphite which has shown exceptional mechanical, optical, and electrical properties enabling it to perform a wide range of different applications. It is the basic building block for many popular carbon containing materials like the 0D “Bucky ball” C60, the popular 1D carbon nanotube, and if layered into a 3D structure, it becomes common graphite. Graphene was originally obtained via mechanical exfoliation of graphite, the so-called ‘scotch tape method’, which however yields graphene flakes of limited sizes. Many different methodologies like sheet extraction from graphene dispersions to epitaxial growth on semiconductors, silicon dioxide, and metals have been used to gain large area sheets of graphene. One interesting method utilizes the sublimation of silicon (Si) from the surface of silicon carbide (SiC), which has generated large scale monolayer and few-layer epitaxial graphene sheets.
Although graphene is an attractive material for the study of quantum electrodynamics, it presents an equal attraction for the generation of a new set of electronic and mechanical devices superior to Si.